Current findings document high rates of comorbid substance use disorder among people with major mental illness. The primary drug abuse in this population is alcohol, followed by cannabis and cocaine. Substance use disorders are frequently not diagnosed in acute care psychiatric settings, leading to a number of adverse consequences such as inappropriate treatment. One reason for this failure to diagnose comorbid substance use disorder in the psychiatrically ill is the lack of sensitive and specific screening instruments and diagnostic procedures suitable for this population. The development of such instruments and procedures poses particular technical challenges because of problems in question comprehension, memory deficits, minimization, and other difficulties that reduce the accuracy of standard alcohol and other drug screens for the severely mentally ill. These problems can be overcome by use of multimodal assessment and the careful adaptation of standard instruments to the specific characteristics of this high-risk population. We propose to develop and validate (a) a brief screen for identifying those psychiatric patients most likely to be abusing alcohol, cannabis and/or cocaine (the Substance Abuse Assessment for the Mentally Ill - Screening Form, SAAMI-S), and (b) a diagnostic battery which utilizes data from standard hospital admissions procedures in combination with specific substance use diagnostic instruments (the Substance Abuse Assessment for the Mentally Ill - Diagnostic Form, SAAMI-D). This battery will be relatively brief, inexpensive and manageable by acutely ill patients in primary care psychiatric settings. It will equal or exceed the overall detection accuracy of such well established instruments as the Alcohol Dependence Scale and the Drug Abuse Screening Test in non-psychiatric populations (i.e., in excess of .85). This accuracy will be achieved by first comparing the most promising diagnostic procedures in a large psychiatric population, and then determining the optimal combination of direct and indirect measures to comprise the brief screen and the multimodal diagnostic battery.